Aedes triseriatus, the treehole mosquito, is the primary vector of La Crosse virus (LAC) the most common form of human encephalitis in the midwest. Both control and monitoring of this mosquito are exceptionally difficult. This research invloves genetic and ecological research on vector competence in an attempt to find an "Achilles' heel". (1) Studies on genetic variability involve establishment of a formal genetics, analysis of gene differences in geographic populations and clarification of relation to sibling species such as Aedes hendersoni, a non-vector of LAC. (2) Ability to transmit LAC varies from 10-90% in different populations; genetics of this trait is under analysis and virus-refractor strains are being developed. Preliminary results show that LAC is deleterious to field populations, leading to selection of resistant forms. (3) A gregarine protozoan pathogen, Ascocystis barretti, infects the intestine of A. triseriatus. There is promising evidence that geographic strains of Ascocystis show different pathogenicity, being especially lethal against Dirofilaria-infested mosquitoes. (4) Egg diapause is controlled by photoperiod, with sensitivity differing in different strains; genetics of this trait is under study, in the hope of using releases for field control. (5) A predatory mosquito larva, Toxorhynchites, is being released in Indiana woodlots for control of A. triseriatus. Five species of Toxorhynchites are being compared in the laboratory for biocontrol potential. (6) Dynamics of field populations is under study in experimental woodlots. Focus is on population size, longevity, vertical distribution, female host choice and male dispersal. (7) Our WHO Reference Center, with 38 species and 200 plus strains of Aedes will continue.